1. Field of the Invention
The present invention relates to an optical pointing device, and more particularly, to a device for obtaining motion data of an object using an image sensor.
2. Description of the Related Art
In general, an image sensor is an optical conversion device including a plurality of photocells, each converting reflected light from an object into an electrical signal corresponding to its intensity.
An optical pointing device converts signals of the above photocells, i.e., electrical signals corresponding to intensities of reflected light from an object into image data for the object through a series of signal processing procedures, compares current image data of the object with previous image data, and calculates and outputs motion data of the object.
FIG. 1 is a block diagram of a conventional optical pointing device. The optical pointing device includes an image sensor 11, an A/D converter 12, a pre-filter circuit 13, an image processor 14, and a shutter control circuit 15.
Respective functions of the blocks shown in FIG. 1 will now be described.
A light source (not shown) for emitting light and a light-source control circuit (not shown) for controlling the light source adjust an amount of the emitted light and illuminate an object (not shown) with the light.
The image sensor 11 includes a pixel array (not shown) for detecting light reflected by the object and outputting an electrical analog signal PSA corresponding to the amount of the light.
The A/D converter 12 receives the electrical analog signal PSA from the image sensor 11 and outputs a digital signal PSAD.
The shutter control circuit 15 receives the output signal PSAD from the A/D converter 12 and outputs a shutter control signal CSH for controlling an electronic shutter (not shown) in the image sensor. The shutter control signal CSH maintains a signal average value of the pixel array of the image sensor at a certain level.
The pre-filter circuit 13 receives the output signal PSAD from the A/D converter 12, and converts the output signal PSAD into 1-bit data, i.e., minimal data structure essential for motion detection according to a predetermined rule.
The predetermined rule may generally be an edge detection algorithm.
The image processor 14 receives an output signal DSO of the pre-filter circuit, and calculates and outputs motion data Vk of the object.
The conventional optical pointing device shown in FIG. 1 necessarily includes the A/D converter 12 and the pre-filter circuit 13 which increase a layout area of the optical pointing device, and in turn, increase a chip size of a semiconductor integrated circuit.
FIG. 2 is a block diagram of another conventional optical pointing device. The conventional optical pointing device includes an image sensor 21, an image processor 24, and a shutter control circuit 25.
Respective functions of the blocks shown in FIG. 2 will now be described.
The image sensor 21 detects light, converts the light into an electrical analog signal corresponding to an amount of the light, and outputs the electrical analog signal as a 1-bit digital signal ISO.
The image processor 24 receives the output signal ISO of the image sensor 21, and derives and outputs motion data Vk of an object. The image processor 24 also outputs a shutter data signal IPO to provide the shutter control circuit 25 and a pixel select signal PS for selecting pixels to provide the image sensor 21.
The shutter control circuit 25 receives the shutter data signal IPO from the image processor 24, and outputs a shutter control signal CSH for controlling an electronic shutter (not shown) in the image sensor 21. That is, the shutter control circuit 25 outputs the shutter control signal CSH for maintaining a signal average value of a pixel array (not shown) in the image sensor 21 at a certain level in response to the shutter data signal IPO.
FIG. 3 is a block diagram of the image sensor of the conventional optical pointing device of FIG. 2.
Referring to FIG. 3, the image sensor 21 includes a plurality of unit pixels 36 to 39. Each of the unit pixels 36 to 39 includes a photocell PC1, a comparator COMP1, and a switch SW1.
The photocell PC1 receives light and generates an analog voltage corresponding to an amount of the light. In response to the shutter control signal CSH, the comparator COMP1 compares an output signal PC001 of the photocell PC1 with an output signal PC002 of the photocell PC2 in the adjacent unit pixel 37 and outputs a 1-bit digital signal COM01.
In this case, a comparator COMP N in a last unit pixel 39 uses any reference voltage VREF1 as a reference voltage.
In response to the pixel select signal PS, the switch SW1 outputs the 1-bit digital signal COM01 from the comparator COMP1 as the output signal ISO of the image sensor 21.
The problem associated with the conventional optical pointing device as shown in FIG. 1 is resolved by using the image sensor 21 including the comparator COMP and the switch SW as shown in FIG. 3. However, in the image sensor 21 of the optical pointing device shown in FIG. 2, inclusion of the comparator COMP and the switch SW in the unit photocells 36 to 39 has a limitation of decreasing the unit photocell size and can not compensate for an offset of each of the photocells PC1 to PC(N) in the unit photocells 36 to 39.